The objective of this research program is to evaluate the alterations of serum lactate dehydrogenas (LDH) isoenzymes of women found to have breast tumors which will be surgically removed. Preliminary observations by the authors have established that an atypical serum LDH isoenzyme pattern is seen in patients with breast carcinoma (1-6). The proposed research program is to evaluate serum LDH isoenzymes in over 800 women with breast tumors both malignant and benign. The LDH isoenzyme patterns in the tumor group will be compared to 200 women free of breast disease in the same age group. The goal of this research is to see if a highly reproducible isoenzyme test can be utilized to determine the presence of breast carcinoma in women.